1. Field of the Invention
This invention relates to battery voltage monitors and charging controllers, particularly those used with a string of series-connected batteries.
2. Description of the Related Art
The use of series-connected rechargeable batteries for energy storage is increasing rapidly. Primary applications include telecommunications power supplies, uninterruptable power supplies, electric utility energy storage and the fast-growing electric vehicle industry. As their usage increases, so do the demands for longer life and easier, cheaper maintenance.
A number of batteries, each consisting of a modular assembly of one or more cells, are connected in series to form a "series string" to achieve a particular string voltage. A number of strings can be connected in parallel to form a "battery pack" to increase the amount of available discharge current. The batteries are usually interconnected both electrically and physically with rigid bus bars, and are charged by flowing a charging current into the string. How rechargeable batteries are charged and discharged can have a significant effect on their lifespan. For example, when valve-regulated lead-acid batteries (VRLA) are being charged, they often suffer a charge deficit that cumulatively increases with each charge. The amount of charge deficit varies from battery to battery--those with a smaller deficit are referred to as "more receptive" to charging current, and those with a large deficit are "less receptive". One way of compensating for a battery's charge deficit is to increase the voltage to which the battery is charged, i.e., the "float" voltage. When a battery's voltage reaches a manufacturer specified float-voltage, it is deemed fully charged. However, increasing the float-voltage to remedy the charge deficit of a less receptive battery can overcharge those batteries in the string that are more receptive. Overcharging causes disassociation of the electrolyte and consequent gas pressurization in a VRLA battery. If the pressure exceeds the relief valve setting, gas escapes and electrolyte is lost, with permanent loss of capacity as the result. The mismatch in charge receptivity grows with the number of charge cycles. When one battery in the string finally suffers an unacceptable loss of capacity, all of the batteries in the string must usually be discarded, even though many of them have substantial useful life remaining.
While overcharging is likely to occur to the more receptive batteries in the string, the less receptive batteries tend to become undercharged. To prevent overcharging, the charge voltage is kept below a limit which is less than the voltage needed to fully charge the less receptive batteries. Chronic undercharging causes sulfation which increases a battery's resistance; this further reduces the charge receptivity of the weaker batteries. When discharged by connecting a load to the string, the battery voltage of the undercharged batteries can reverse. This irreversibly damages the batteries, and can escalate into an explosion or fire if discharge continues.
Temperature must also be taken into account when charging a battery. When charging is near completion, a battery's voltage should be greater than a particular float-voltage limit and less than an over-voltage limit. These limits are typically specified by the manufacturer at a particular nominal temperature. Also specified is a temperature compensation factor that dictates how the float-voltage and over-voltage limits shift when the battery's temperature exceeds the nominal temperature. Thus, a battery cannot be optimally charged without accurately monitoring its temperature as well as its voltage.
One system devised for use with rechargeable batteries is described in U.S. Pat. No. 5,438,250 to Retzlaff. A length of conductor is connected from each battery terminal in a string to a complex relay switching network. Voltage monitoring circuitry and a floating power supply are sequentially connected to each battery. Since large potential differences are possible between conductors, and the batteries and conductors are often located in explosive areas, this approach can be dangerous. The large number of conductor cables required for a lengthy battery string also make the system prone to noise from nearby electrical or magnetic fields. Also, the system does not monitor the temperature of individual batteries; thus, a battery's float-voltage and over-voltage limits cannot be accurately temperature compensated.
An approach taken to solve the overcharging problem in the functionally similar case of monitoring and charging the individual cells that make up a single battery is described in U.S. Pat. No. 4,614,905 to Petersson et al. This patent discloses an autonomous linear shunt current regulator connected to an individual cell of a battery that prevents the cell's voltage from exceeding an over-voltage limit when charge current is flowed through the cell. If a cell's voltage exceeds a preset limit, a transistor and resistor connected across the cell are made to shunt an amount of current necessary to maintain the cell's voltage close to the limit. This approach is satisfactory unless the amount of shunt current needed to adequately reduce the cell's voltage exceeds the current limit of the regulator--the occurrence of this condition is not addressed. The device also fails to address the undercharging and voltage reversal problems, as the shunt current limitation may prevent the least receptive cells in a battery from being sufficiently charged before a charge cycle must be terminated to prevent a more receptive cell from exceeding the over-voltage limit. There is also no discussion of a system that can control the charge cycle for all cells in a battery or all batteries in a string.
E.P. 0 277 321 to Geuer, et. al., describes a system that monitors the voltage, current and temperature of individual batteries in a string by converting sensor values to a digital format that is then sent over a network to a central processor. The processor is burdened with extracting the essential information, i.e., the voltage, current, and temperature values with respect to prescribed limits, from vast amounts of raw data. This system only addresses the monitoring of batteries in a string, with no charging control capabilities. The overcharging, undercharging and cell reversal concerns described above are not addressed.
A system is needed that can safely, accurately and economically monitor the voltage and temperature status of each individual battery in a string, and can individually regulate the charge current to each battery.